This invention relates to systems and methods of forming ferroelectric Pb(Zr,Ti)O3 (PZT) films, including ferroelectric PZT films for use in ferroelectric random access memory devices.
Today, several trends exist in the semiconductor device fabrication industry and the electronics industry that are driving the development of new material technologies. First, devices are continuously getting smaller and smaller and requiring less and less power. A reason for this is that more personal devices are being fabricated which are very small and portable, thereby relying on a small battery as its supply source. For example, cellular-phones, personal computing devices, and personal sound systems are devices that are in great demand in the consumer market. Second, in addition to being smaller and more portable, personal devices are requiring more computational power and on-chip memory. In light of these trends, there is a need in the industry to provide a computational device that has a fair amount of memory and logic functions integrated onto the same semiconductor chip. Preferably, this computation device will include a non-volatile memory so that if the battery dies, the contents of the memory will be retained. Examples of conventional non-volatile memories include electrically erasable, programmable read only memories and flash EEPROMs.
A ferroelectric memory (FeRAM) is a non-volatile memory that utilizes a ferroelectric material as a capacitor dielectric situated between a bottom electrode and a top electrode. Ferroelectric materials, such as SrBi2Ta2O9 (SBT) and Pb(Zr,Ti)O3 (PZT), are being used in the fabrication of a wide variety of memory elements, including ferroelectric random access memory (FeRAM) devices. In general, ferroelectric memory elements are non-volatile because of the bistable polarization state of the material. In addition, ferroelectric memory elements may be programmed with relatively low voltages (e.g., less than 5 volts) and are characterized by relatively fast access times (e.g., less than 40 nanoseconds) and operational robustness over a large number of read and write cycles. These memory elements also consume relatively low power, may be densely packed, and exhibit radiation hardness.
Recent efforts to develop fabrication processes for ferroelectric materials have focused on integrating FeRAM technology with semiconductor integrated circuit technology. Accordingly, such efforts have focused on scaling FeRAM capacitor areas, cell sizes and operating voltages downward in accordance with the scale of current integrated circuit dimensions. In addition to small lateral dimensions (i.e., dimensions parallel to the film surface), the ferroelectric dielectric must be relatively thin and must have a relatively low coercive field to achieve FeRAM devices having low operating voltages.
Recently, PZT has been demonstrated to be scalable to relatively small lateral dimensions and low operating voltages. For example, International Patent Publication No. WO 00/49646 discloses a process for forming a scalable PZT material by liquid delivery metalorganic chemical vapor deposition (MOCVD) without PZT film modification techniques, such as acceptor doping or use of film modifiers (e.g., Nb, Ta, La, Sr, Ca, and the like). In accordance with this process, liquid precursor solutions of the component metals are mixed and flash vaporized. The resulting source reagent vapor is introduced into a chemical vapor deposition chamber where the PZT film is deposited on a substrate. In one embodiment, the metalorganic precursors are lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate) (hereinafter xe2x80x9cPb(thd)2xe2x80x9d) as a Pb precursor, titanium bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate) (hereinafter xe2x80x9cTi(O-i-Pr)2,(thd)2xe2x80x9d) as a Ti precursor, and zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate) (hereinafter xe2x80x9cZr(thd)4xe2x80x9d) as a Zr precursor In another embodiment, the lead precursor is lead bis(2,2,6,6-tetramethyl-3,5-heptanedionate) N, Nxe2x80x2, Nxe2x80x3-pentamethyl diethylenetriamine (hereinafter xe2x80x9cPb(thd)2pmdetaxe2x80x9d) and the zirconium precursor is zirconium bis(isopropoxide)bis(2,2,6.6-tetramethyl-3,5-heptanedionate) (hereinafter xe2x80x9cZr(O-i-Pr)2(thd)2xe2x80x9d). The solvent media used in the liquid delivery MOCVD process is selected to be compatible with the specific metalorganic precursors used for forming the PZT thin film materials and efficacious in the constituent liquid delivery and CVD process steps. Illustrative multi-component solvent compositions include: tetrahydrofuran: isopropanol: tetraglyme in a 8:2:1 volume ratio; octane: decane: polyamine in a 5:4:1 volume ratio; and octane: polyamine in a 9:1 volume ratio. According to the WO 00/49646 patent publication, the resulting PZT material is pulse length scalable or E-field scalable, or both, and is useful for ferroelectric capacitors having dielectric thicknesses that range from about 20 nanometers to about 150 nanometers and having lateral dimensions that extend down to as small as 0.15 micrometers.
Chemical vapor deposition (CVD) is a particularly attractive method for forming thin PZT films because CVD is readily scaled up to production runs and because CVD technology is sufficiently mature and developed that CVD may be applied readily to new film processes. In general, CVD requires that the element source reagents (i.e., the precursor compounds and complexes containing the elements or components of interest) must be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The elemental component source reagents should decompose in the CVD reactor for deposition on the desired substrate surface at the desired growth temperatures. Premature gas phase reactions leading to particulate formation should be avoided. In addition, the source reagents should not decompose in the transport lines before reaching the reactor deposition chamber. In sum, in order to deposit CVD films having desirable properties, the stoichiometry and other process conditions must be controlled for a given baseline chemistry to create a transport window that enables component materials to combine on a substrate in a desired way.
The invention features improved methods of forming PZT thin films that are compatible with industry-standard chemical vapor deposition production techniques. The invention enables PZT thin films having thicknesses of 70 nm or less to be fabricated with high within-wafer uniformity, high throughput and at a relatively low deposition temperature.
In one aspect, the invention features a method of forming a ferroelectric PZT film on a substrate. In accordance with this method, a premixed source reagent solution comprising a mixture of a lead precursor, a titanium precursor and a zirconium precursor in a solvent medium is provided. The source reagent solution is vaporized to form a precursor vapor. The precursor vapor is introduced into a chemical vapor deposition chamber containing the substrate.
Embodiments of the invention may include one or more of the following features.
The zirconium precursor preferably comprises Zr(OiPr)2(thd)2 or Zr(thd)4 or Zr(OtBu)2(thd)2. In one embodiment, the lead precursor is Pb(thd)2(pmdeta), the zirconium precursor is Zr(OiPr)2(thd)2, and the titanium precursor is Ti(OiPr)2(thd)2. The lead precursor, the titanium precursor and the zirconium precursor preferably have a combined concentration between about 0.05 M and about 1.0 M in solution. The source reagent solution preferably is characterized by lead, zirconium and titanium concentrations between about 5% and 95%.
In some embodiments, an oxidizing co-reactant gas comprising 5-100% N2O and, more preferably 50-75% N2O, is introduced into the chemical vapor deposition chamber. The oxidizing co-reactant gas also may include O2 or O3, or both.
In some embodiments, a second source reagent solution comprising a second premixed mixture of the lead precursor, the titanium precursor and the zirconium precursor in the solvent medium is provided. The first source reagent mixture preferably is different from the second source reagent mixture. The first and second reagent solutions are mixed to form a precursor solution, and the precursor solution is vaporized to form the precursor vapor. In one embodiment, the first and second source reagent solutions preferably are characterized by a lead concentration in a range of about 28-65%, a zirconium concentration in a range of about 14-29%, and a titanium concentration in a range of about 20-43%.
The solvent medium preferably comprises an octane-based solvent.
The source reagent solution may be vaporized at a temperature in the range of about 180-210xc2x0 C. During deposition, the chemical vapor deposition chamber preferably is maintained at a pressure in a range of about 0.5-10 torr and, more preferably, in a range of about 0.5-4 torr and, still more preferably, at a pressure of approximately 4 torr. The source reagent solution preferably is selected to obtain a precursor vapor having a Zr/(Zr+Ti) ratio in the range of about 0.05-0.70 and a Pb/(Zr+Ti) ratio in the range of about 0.3-3.0.
The substrate preferably is preheated during a preheating period prior to disposing the substrate on the susceptor. The preheating period may be about 5-30 seconds long. The preheated substrate may be deposited on a heated susceptor during a heating period prior to formation of the PZT film on the substrate. The heating period may be about 30-60 seconds long or longer. A flow of a purge gas may be provided to reduce film depositions on the susceptor and chamber wall surfaces. In some embodiments, in addition to the purge gas flow, the pre-heating and heating process steps may be performed in a gas flow that includes a combination of one or more of the following gases: O2, N2O, O3, and an inert gas (e.g., He, N2, or Ar).
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.